1. Field of the Invention
The present invention relates to a process for the purification of a low-calorie soluble glucose polymer. More precisely, it relates to a process for the purification of a polydextrose or of a polyglucose or of a pyrodextrin, consisting in treating the polydextrose, or the polyglucose or the pyrodextrin, using glucose oxidase and an anion exchanger in hydroxyl OH.sup.- form.
2. Description of the Prior Art
It is known that glucose, optionally mixed with polyols, may be polymerized under hot conditions in the presence of inorganic or carboxylic acids, according processes which have been described, for example, in U.S. Pat. Nos. 2,436,967, 2,719,179 and 4,965,354 as regards the inorganic acids or U.S. Pat. Nos. 3,766,165 and 5,051,500 as regards the carboxylic acids.
The soluble polyglucoses obtained, composed mainly of 1-6 glucose-glucose linkages and of atypical 1-2 and 1-3 glucose-glucose linkages, are difficult to digest.
Their low caloric value, which is generally thought to be in the region of 4.18 kilojoules/gram (equivalent to 1Cal/g), and their technological properties which are close to those of sucrose make them candidates for the replacement of this sugar as filling agents, inert products which are very much sought after as dietary fibre.. They also possess physical properties which enable them to substitute for certain fats and thereby to reduce the caloric power thereof drastically.
However, the organoleptic properties of the polymers thus obtained do not permit them to find the widest possible use in food products.
The reason for this is that these crude polymers are still coloured or acidic or bitter, which does not allow them to be used in the total replacement of sugar or of fats and which limits their application to products such as fruit juices, which already have a certain coloration, acidity and bitterness.
In addition, as a result of the presence of free glucose, and to a lesser extent of free sorbitol, in these crude polymers, these products are not as low in calories as possible.
The persistence of free glucose or of free sorbitol in these polymers also gives rise to other drawbacks, such as the existence of a sweet taste, which is incompatible with fats, and results, moreover, in a reducing of the equilibrium viscosity and relative humidity of the polymers obtained or of the products in which they are incorporated as sugar substitutes.
Numerous attempts have already been made to provide these polymers with the qualities which they lack.
Thus, it has been proposed to purify polydextrose (glucose polymer, or polyglucose, obtained most generally condensation of glucose in the presence of sorbitol and citric acid) by decolouring it using peroxides and then by precipitating it via organic solvents, so as to liberate it from the free glucose, sorbitol and citric acid, as well as from levoglucosan which would impart a bitter taste thereto. An account of this process has been given in U.S. Pat. No. 4,622,233. However, the amounts of solvent to be used are enormous and the process is thus not satisfactory from the points of view of economy and safety.
Another process for purifying polydextrose, an account of which is given in European Patent Application No. 289,461, consists in removing the coloured and bitter impurities therefrom by extraction using organic solvents, but without precipitating the polymer. In this case also, the amounts of solvent to be used are enormous, and this process therefore suffers from the same drawbacks of economy and safety.
European Patent Application No. 342,156 proposes to remove the low-molecular-weight compounds present in the crude polydextrose, which are essentially glucose, sorbitol,, citric acid and levoglucosan, by reverse osmosis and diafiltration. The process is expensive since, in this case., it requires enormous amounts of water in order to perform the osmosis and then the diafiltration and, in addition, the strongly coloured large molecules are not removed and contribute towards the poor appearance of the product obtained.
European Patent Application No. 380,248 proposes to treat polydextrose with adsorbent or weak anionic resins so as to lower the free and esterified citric acid content thereof and claims thus to enhance the taste thereof. The product obtained still contains, however, low-molecular-weight caloric compounds such as glucose and sorbitol, and it is not rid of all bitterness or of coloration.
European Patent Application No. 458,748 proposes a similar process to the above, using anionic celluloses, and also leads to a product having the same drawbacks.
European Patent Application No. 473,333, via a 20 process using several ion-exchange resins of differing functionalities, claims that it lowers the residual levels of free and esterified citric acid in the polydextrose to the limit of detection, and asserts that a product of reduced bitterness is thus obtained. However, the product obtained still contains low-molecular-weight caloric compounds such as glucose and sorbitol and it is still coloured. This specific polydextrose is marketed by the company Pfizer under the brand name LITESSE.RTM. II.
PCT Patent Application WO 92/12179, via a process of molecular sieving on cationic resins that retains the compounds of low molecular weight, makes it possible to obtain a product similar to that obtained by the process outlined in European Patent Application No. 342,156 already cited above. However, just as in this patent application, the process is expensive since molecular sieving requires a large amount of water. Moreover, the strongly coloured large molecules are not separated from the polymer either, and contribute towards the poor appearance of the product obtained.
PCT Patent Application WO 92/14761 proposes a polydextrose or a polyglucose (low-calorie soluble glucose polymer obtained by the action of inorganic acids on glucose) containing only a small amount of reducing glucose groups. Such a product is obtained especially by catalytic hydrogenation of a polydextrose or polyglucose solution. The product obtained is largely colourless, but is not as low in calories as possible since it contains sorbitol originating from the hydrogenation of the free glucose contained in the polydextrose or the polyglucose as well as the free sorbitol already present in the polydextrose.
European Patent application No. 368,451 proposes treat a pyrodextrin with a starch hydrolysis enzyme such as alpha-amylase, for the purpose of deodorizing it and of removing the unpleasant taste therefrom so that it may serve as dietary fibre. A pyrodextrin is a low-calorie soluble glucose polymer obtained by dry-roasting starch containing, as in polyglucose or polydextrose, atypical 1-2 and 1-3 glucose-glucose linkages, but mainly containing standard starch 1-6 linkages and especially 1-4 linkages, making this product digestible to an extent of about 50%, which thus imparts a caloric value of 8 Kjoules/gram (equivalent to 2 Cal/g).
U.S. Pat. 5,094,951 describes the production of glucose oxidase by a genetically recombined yeast: Saccharomyces cerevisiae. Glucose oxidase (EC 1.1.3.4) is the enzyme which catalyses the oxidation of glucose into gluconic acid with concomitant production of hydrogen peroxide. This enzyme has many industrial applications, among which there may be noted its use for removing glucose from egg white or from egg yolk before drying them, the aim being to avoid Maillard browing reactions, for removing residual glucose from syrups with a high fructose content, for removing dissolved oxygen contained in drinks, moist food products, perfumes when these are contained in leakproof packaging, and for assaying glucose in industrial products and in biological fluids such as blood and urine.
Bearing in mind that there is a growing interest in good-quality soluble food fibres which are largely colourless, as taste-free as possible and also as low in calories as possible, the Applicant Company has carried out much research with the aim of developing an economic process which enables such a quality of fibres to be obtained.